The invention relates to a process for production of graft copolymers and more specifically to a continuous process for the grafting of styrene and acrylonitrile monomers onto a rubbery polymeric latex to form an acrylonitrile-butadiene-styrene copolymer (ABS).
By polymerizing a mixture of styrene and acrylonitrile in the presence of, for example, a polybutadiene rubber, compositions are obtained which are comprised of said rubber and acrylonitrile-styrene copolymers, a part of these copolymers being grafted onto the rubber. ABS resins have good mechanical properties, particularly a high impact resistance, when the rubber has grafted thereon at least 10% of its own weight of the styrene-acrylonitrile copolymers, with the remainder of the copolymers forming a continuous phase wherein the particles of grafted rubber are dispersed.
The production of ABS resins by grafting a mixture of styrene and acrylonitrile monomers in the presence of a polybutadiene rubber is generally a batch-wise process. In recent years, however, many efforts have been devoted to the production of ABS by a continuous process.
According to one of these continuous processes, the rubbery latex is admixed with the monomers and the mixture is allowed to stand for a period of time which may reach several hours. The rubber particles absorb the monomers, and this period is called the imbibition period. Thereafter, the polymerization is carried out in two steps. The amount of monomer which is converted to polymer in the first step generally does not exceed 65% by weight of the total monomers employed. When the polymerization is carried out in one step, the properties of the ABS resin are poorer, especially the impact resistance and the melt flow index. See British Patent Specification No. 1,373,089.
According to a further continuous process, the latex is first imbibed with some or all of the styrene to be grafted, in the absence of acrylonitrile, and this mixture is then imbibed with acrylonitrile together with the balance, if any, of the styrene. Thereafter, graft polymerization of the monomers is initiated. The impact resistance of the thus-obtained ABS graft copolymer is higher in comparison to the impact resistance of a similar ABS resin prepared without the first imbibition by styrene in the absence of acrylonitrile. See U.S. Pat. No. 3,855,355.
These processes require at least three steps, and the production time for the ABS resins (imbibition+graft polymerization time) is very long where it is desired to obtain an optimum degree of grafting. Moreover, several separate reactors have to be connected in series, with careful control of the rates of introduction and of withdrawal. As a result, these processes require investment of large sums and are not very suitable for an efficient production of ABS resins on a commercial scale.
According to the prior art, the rubber particles must absorb monomers (styrene and acrylonitrile) before the polymerization step, the absorbed monomers being then polymerized within the swollen rubber particles. Without the imbibition step, the resulting ABS resins have poor properties. However, imbibition must be carried out cautiously, since absorption of too large an amount of monomers likewise results in ABS resins having poor properties.
In fact, the prior processes for manufacturing ABS resins comprise one or more steps to allow diffusion of styrene and/or acrylontriile into the rubber particles and then one or more polymerization steps wherein graft polymerization occurs within the swollen particles, always requiring at least three steps.